correct bugs in dependency specifications and fix easyconfigs unit tests

This PR resolves bugs in the dependency specifications in easyconfig files caused by an underspecified/ambiguous format (see https://github.com/hpcugent/easybuild-framework/issues/686).

Dependency specifications in which the toolchain was glued to the `version` (or `versionsuffix`) is now done properly, e.g.:

```python
('OpenMPI', '1.6.4', '', ('GCC', '4.7.2'))
```

instead of (**incorrect**):

```python
('OpenMPI', '1.6.4-GCC-4.7.2')
```

or (also **incorrect**):

```python
('OpenMPI', '1.6.4', '-GCC-4.7.2')
```

This depends on https://github.com/hpcugent/easybuild-framework/pull/679.